Hot rolled steel plate has generally been produced by use of a reversing plate mill rolling from "pattern" slabs to plate. Reversing plate mills specifically dedicated to rolling "pattern" slabs to plate are generally used for producing wider and thicker plate as compared to a hot strip mill product. It is the usual practice to produce plate on a single stand or a two-stand reversing mill. Each combination of thickness, width, and length of plate rolled from the mill requires a properly proportioned "pattern" slab with the appropriate volume of metal. The slabs are reduced to plates by passing them back and forth through the mill. It is usual to cross roll a slab to achieve the desired plate width. Thereafter, the rolled plates are flattened hot on a leveling machine, transferred to a cooling bed for cooling and subsequently side sheared and end sheared to finish plate dimensions. This reduction normally takes place on a four-high hot reversing plate mill, although it is also common to utilize an additional two-high or four-high hot reversing mill upstream of the four-high reversing mill to increase productivity by having two slabs on line at a time.
The use of only "pattern" slabs in the mill is problematic. Since each plate size is made from a corresponding pattern slab, the reheat furnace must accommodate a wide range of slab sizes to produce the product mix, thereby making heating efficiency and uniformity more difficult. Further, the slab producing facility, whether it be a continuous caster or a blooming or slabbing mill must turn out a large number of small size slabs for subsequent processing into the plates. For example, a typical 112-inch wide plate mill requires approximately 30,000 slabs for each 100,000 tons of plate production. This is further compounded by the typical market demand for carbon steel plate wherein approximately 50% of the demand is for plate which is one-half inch thick or less. To meet this market, the reversing plate mill must roll many slabs (2 to 3 tons) at a resultant low production rate and with a low product yield.
The slabs must be obtained from a slabbing mill or continuous caster, cut to "pattern" dimensions, marshalled in the plate mill slab yard and charged into the plate mill furnace in the proper rolling sequence. Therefore, in addition to low production rate and yield, substantial costs are involved in the repeated handling and marshalling of many small slabs.
One attempt to improve the productivity of reversing plate mills involves a process for continuous conversion of an extra large slab, at least 32 tons in weight, into a number of plates on a plate mill. The process involves heating the slab to a desired rolling temperature. The heated slab is then passed back and forth through a hot reversing mill to obtain a workpiece of desired intermediate thickness. After each pass through the hot reversing mill, the workpiece is coiled and heated within a coiler furnace until the workpiece has been reduced to a desired plate thickness. The workpiece is then decoiled from one of the coiler furnaces and fed to an in-line shear where it is sheared into a number of plates of desired length.
By using only extra large slabs, the productivity of the reversing plate mill is increased and manufacturing costs are lowered. This is achieved because the reversing plate mill requires a smaller building, and less equipment, manpower and services than an equivalent two-stand facility and, thus, requires appreciable lower capital expenditure. In addition, by processing only extra large slabs, a substantial increase in product yield is achieved, which lowers unit manufacturing costs and considerable raw material, energy and other resources.
While rolling only large slabs minimized the problems associated with rolling only pattern slabs, the problem of storing the finished excess product in inventory offset some of the savings by rolling large slabs. U.S. Pat. No. 4,658,363 attempts to reach a middle ground between rolling only pattern slabs or extra large slabs, by rolling both extra large slabs, on the order of 30 to 40 tons each, and pattern slabs, on the order of 2 to 11 tons each. A decision is made to process extra large slabs, to process pattern slabs or a combination of both extra large and pattern slabs to supply the plate requirements for the next determinable period of time, called the horizon period in the '363 patent. That is, a decision is made to either roll the pattern plates in flat form as a series of single plates from a corresponding series of pattern slabs or to roll from a single extra large slab a coil of metal to the described dimensions of the total number of pattern plates and then shear the pattern plates from the coil. Several plates are rolled in coil form from a single slab when more than 60% of the coil can be utilized for the ordered pattern plates during the horizon period. The remaining portion of the coil, if any, is stored in inventory. Otherwise pattern slabs are used to satisfy the pattern plate requirements. The value of 60% is considered as a break-even point in which the savings due to an increase in yield are offset by losses due to the storage of the finished product in the inventory. While both pattern and extra large slabs are rolled, only a single thickness product is rolled from each slab.
The process of rolling both pattern slabs and large slabs in a reversing plate mill is problematic in that it is still necessary to select and feed the appropriately sized slabs to the reversing plate mill. Further, where a decision is made to roll a large slab, but the slab will produce a number of plates exceeding the number ordered, it is necessary to place the remaining plates in inventory. Thus, there are losses due to the costs of storing finished product in inventory.
Hence, a need has arisen for a method of controlling the hot reversing mill wherein a significant amount of excess finished product is not generated and, therefore, does not have to be inventoried. A need has also arisen for a method for controlling a hot reversing mill wherein the particular size of the slab to be rolled is generally uniform to reduce the costs associated with handling and feeding different-sized slabs into the mill. The present invention satisfies these needs.
The present invention overcomes the shortcomings of the state of the art and achieves further advantages which will be apparent after reviewing the foregoing and following specification.